1. Field of the Invention
This invention relates to lens systems for projecting an image onto a focal plane and more particularly to the field of scanners for periodically and reciprocally translating the image on the image plane between a first and second limit position. The invention scanner is particularly adapted for use in light weight portion FLIR (Forward Looking Infrared) surveillance systems and is typically employed with an objective lens system for scanning an image on a sensitive image plane.
2. Description of the Prior Art
Various optical scanner devices are shown in the art for deflecting or scanning a beam of light on an image plane or sensitive focal plane. These devices range from a simple mirror or refractive element rotating about an axis perpendicular to the optical axis of the system to other more sophisticated systems including those using counter-rotating wedges as described in the "Fundamentals of Optics" by Jenkins and White, McGraw-Hill, 1957, pages 23 and 24. The counter-rotating wedge scanner is widely accepted in the optics field for applications requiring precise control of angle of deflection and beam control. Another suggested wedge-prism optical scanner is described in U.S. Pat. No. 3,881,802, issued May 6, 1975, to U. V. Helava and describing a wedge-prism scanner embodying only one rotating wedge.
One problem associated with scanners known in the art is that they translate the image on the image plane with a sinusoidal velocity on the image plane. Another problem associated with the counter-rotating wedge beam scanner is that the wedges must be rotated in opposite direction with perfectly matched angular velocities. Variations in the respective angular velocities of each of the respective prisms will result in a deflection error producing an orbital or elliptical motion of the image on the image plane.
Another problem typically associated with counter-rotating wedge beam scanners is the requirement for precision in manufacture of the respective rotating wedges for those applications requiring relatively constant image velocity on the image plane. Another and most important problem typically associated with the counter-rotating wedge scanner when used to scan an image on an image plane with relatively constant image velocity on the image plane is low optical scan efficiency. The low scan efficiency results from the fact that the image translates on the image plane with a velocity relative to the image plane varying as a sinusoidal function. To obtain a relative linear velocity, the application must restrict the usable range of the scan from typically minus thirty degrees to plus thirty degrees of prism rotation for each half cycle of prism rotation. The image or the signals derived from the image on the image plane just be blanked or remain unused during the remainder of the counter-rotating wedge prism angle of rotation; thereby, reducing the information available at the focal plane by the ratio of blanking time to half cycle rotation time.